Spallation-altered accreted compositions for X-ray bursts: Impact on ignition conditions and burst ashes

TL;DR

This study investigates how spallation alters the composition of accreted material on neutron stars, affecting X-ray burst predictions by replenishing CNO elements and changing burst ashes and crust structure.

Contribution

It introduces a spallation model that accounts for cascading destruction, revealing enhanced CNO element survival and altered burst ash compositions compared to traditional models.

Findings

Spallation replenishes CNO elements in accreted material.

Metal distribution varies with mass accretion rate.

Altered composition impacts neutron star crust structure.

Abstract

Dependable predictions of the X-ray burst ashes and light curves require a stringent constraint on the composition of the accreted material as an input parameter. Lower metallicity models are generally based on a metal deficient donor and all metals are summed up in CNO abundances or solar metal distribution is assumed. In this work, we study the alteration of accreted composition due to spallation in the atmosphere of accreting neutron stars considering a cascading destruction process. We find that the inclusion of the cascading process brings the replenishment of CNO elements and overall survival probability is higher compared to isolated destruction of CNO elements. Spallation model provides the distribution of metals as a function of mass accretion rate. Multi-zone X-ray burst models calculated with reduced metallicities have enhanced abundances for high-mass nuclei in X-ray burst ashes. The increased metallicity due to the replenishment of CNO elements changes the composition of burst ashes compared to lower metallicity conditions. This will modify the thermal and compositional structure of accreted neutron star crusts.